In a twin roll caster, molten metal is introduced between a pair of counter-rotated, internally cooled casting rolls so that metal shells solidify on the moving roll surfaces, and are brought together at the nip between them to produce a solidified strip product, delivered downwardly from the nip between the casting rolls. The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. The molten metal is poured from a ladle through a metal delivery system comprising a tundish and a core nozzle located above the nip to form a casting pool of molten metal, supported on the casting surfaces of the rolls above the nip and extending along the length of the nip. This casting pool is usually confined between refractory side plates or dams held in sliding engagement with the end surfaces of the rolls so as to dam the two ends of the casting pool against outflow. The cast strip is typically directed to a hot rolling mill where the strip is hot reduced by 10% or more.
In certain steel applications, medium and high carbon pearlitic steels have been desired, such as for banding or strapping, and certain applications such as wall-ties for construction needing high strength and controlled toughness. Pearlite typically offers increased strength with reduced impact toughness. In the past, pearlitic microstructures were obtained by hot rolling and slow cooling medium and high carbon steel strip having thicknesses greater than about 3.0 millimeter. Obtaining thinner strip such as 1.0 to 1.5 millimeter required multiple rounds of cold rolling and annealing that coarsened if not eliminated the pearlite, reducing the strength of the steel.
A hot rolled steel strip is disclosed made by the steps comprising:                assembling an internally cooled roll caster having laterally positioned casting rolls forming a nip between them, and forming a casting pool of molten steel supported on the casting rolls above the nip and confined adjacent the ends of the casting rolls by side dams, the molten steel of such composition that hot rolled thin cast strip produced has a composition comprising by weight, greater than 0.25% and up to 1.1% carbon, between 0.4 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.01% aluminum,        counter rotating the casting rolls to solidify metal shells on the casting rolls as the casting rolls move through the casting pool, and        forming from the metal shells downwardly through the nip between the casting rolls a steel strip,        hot rolling the steel strip such that mechanical properties at 10% and 35% reduction are within 10% for yield strength, tensile strength and total elongation; and        coiling the hot rolled steel strip at a temperature between 550 and 750° C. to provide a majority of the microstructure comprising pearlite, along with bainite and acicular ferrite in the microstructure.        
Alternatively, the step of hot rolling may be such that mechanical properties at 15% and 35% reduction are within 10% for yield strength, tensile strength and total elongation. In another alternative, the mechanical properties are within 10% throughout the range from 15% to 35% reduction for yield strength, tensile strength and total elongation. Alternatively, mechanical properties may be within 10% throughout the range from 10% to 35% reduction for yield strength, tensile strength and total elongation.
The molten steel cast may have a free oxygen content between 5 and 50 ppm or content between 25 and 45 ppm. The total oxygen content may be more than 20 and typically less than 100 ppm.
The molten steel may have a composition such that the manganese content of the composition of the hot rolled steel strip is between 0.9 and 1.3% by weight. Alternatively or in addition, the molten steel may have a composition such that the niobium content of the composition of the hot rolled steel strip is between about 0.01% and 0.2%. Alternatively or in addition, the composition of molten steel may be such that the composition of the hot rolled steel strip may have a composition comprising at least one element selected from the group consisting of molybdenum between about 0.05% and about 0.50%, vanadium between about 0.01% and about 0.20%, and a mixture thereof. The hot rolled steel strip may have a tensile strength of at least 800 MPa after hot rolling reductions of 15% and 35%.
The steps of making the hot rolled steel strip may include hot dip coating the hot rolled steel strip to provide a coating of zinc or a zinc alloy.
Alternatively, a hot rolled steel strip may be made by the steps comprising:                assembling an internally cooled roll caster having laterally positioned casting rolls forming a nip between them, and forming a casting pool of molten steel supported on the casting rolls above the nip and confined adjacent the ends of the casting rolls by side dams, the molten steel of such composition that hot rolled thin cast strip produced has a composition comprising by weight, greater than 0.25% and up to 1.1% carbon, between 0.5 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.01% aluminum,        counter rotating the casting rolls to solidify metal shells on the casting rolls as the casting rolls move through the casting pool, and        forming from the metal shells downwardly through the nip between the casting rolls a steel strip,        hot rolling the steel strip such that mechanical properties at 10% and 35% reduction are within 10% for yield strength, tensile strength and total elongation; and        coiling the hot rolled steel strip at a temperature between 200 and 550° C. to provide a majority of the microstructure comprising bainite, martensite, and acicular ferrite, and having less than 5% polygonal ferrite in the microstructure.        
Alternatively, the step of hot rolling may be such that mechanical properties at 15% and 35% reduction are within 10% for yield strength, tensile strength and total elongation. In yet another alternative, the mechanical properties are within 10% throughout the range from 15% to 35% reduction for yield strength, tensile strength and total elongation. Alternatively, mechanical properties may be within 10% throughout the range from 10% to 35% reduction for yield strength, tensile strength and total elongation.
The molten steel cast may have a free oxygen content between 5 and 50 ppm or content between 25 and 45 ppm. The total oxygen content may be more than 20 and typically less than 100 ppm.
The molten steel may have a composition such that the manganese content of the composition of the hot rolled steel strip is between 0.9 and 1.3% by weight. Alternatively or in addition, the molten steel may have a composition such that the niobium content of the composition of the hot rolled steel strip is between about 0.01% and 0.2%. Alternatively or in addition, the composition of molten steel may be such that the composition of the hot rolled steel strip may have a composition comprising at least one element selected from the group consisting of molybdenum between about 0.05% and about 0.50%, vanadium between about 0.01% and about 0.20%, and a mixture thereof. The hot rolled steel strip may have a tensile strength of at least 1100 MPa after hot rolling reductions of 15% and 35%. Alternatively, the hot rolled steel strip may have a tensile strength between about 1100 and 1700 MPa after hot rolling reductions of 15% and 35%.
The steps of making the hot rolled steel strip may include hot dip coating the hot rolled steel strip to provide a coating of zinc or a zinc alloy.